Why is the network transfer so slow when sending many small DICOM images?

The reason might be the so-called Nagle algorithm, which allows for improving the efficiency of TCP/IP networks by reducing the number of packets that need to be sent over the network. As a drawback, this algorithm might also decrease the speed of the network transfer on some systems, e.g. when sending many small DICOM images. Therefore, this algorithm can be switched on and off.

Starting with DCMTK 3.6.2, the Nagle algorithm is not disabled by default since this does not seem to be appropriate (anymore) for most modern operating systems. In order to change this default, the environment variable NO_TCPDELAY can be set to "1" (see envvars.txt for details). Alternatively, the macro DISABLE_NAGLE_ALGORITHM can be defined to change this setting at compilation time (see macros.txt for details).

Earlier versions of the DCMTK disabled the Nagle algorithm by default, and the macro DONT_DISABLE_NAGLE_ALGORITHM could be used to change this setting at compilation time.